Doesn't make sense for a belt to squeal only when shifting, are you sure it is a belt? Is the belt old or dryed out and cracked looking?
Try using a bar of soap and while the engine is running at an idle, CAREFULLY, hold the bar of soap up against the belt to "coat" it. If it is a belt, this should stop the squeal temporarily. If it does, then replace the belt. If it doesn't, then it isn't your belt.
Tremec TKO, T45 & T56 Transmission Systems
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TKO, T45, 3650, T5 & T56 Transmission Systems
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